In the drilling of a well by the rotary method, a drilling fluid is often used which is generally an aqueous clay suspension and which may also contain weighting agents to increase the hydrostatic head and concentrated colloidal suspending and conditioning agents.
The drilling fluid serves to bring cuttings to the surface, to cool the bit and to keep the oil, gas and water confined to their respective formations during the drilling process. For these functions, it is necessary that the drilling fluid be of pumpable viscosity, have sufficient carrying capacity to bring cuttings to the surface, and yet be fluid enough to release cuttings and entrained gas at the surface.
A highly important property of drilling fluids, also referred to as drilling muds, is the ability to form a low permeability filter cake upon the permeable walls of the borehole, thus inhibiting the ingress of drilling fluid into the formation. Excessive fluid loss from the drilling fluid can cause severe problems. For example, the buildup of filter cake can become so thick that the drill pipe becomes stuck. Also, this buildup may cause great difficulty when withdrawing pipe from the hole. High water losses to penetrated formations can cause sloughing and caving in of shale formations. In addition, electrical logging of the well can be adversely influenced due to the mud filtrates, etc.
Drilling fluids are frequently characterized by procedures established by the American Petroleum Institute (API) for the measurement of (1) plastic viscosity, (2) yield point, (3) initial gel strength, (4) ten minute gel strength, and (5) water loss which relates to the sealing off of the penetrated formation by a filter cake. Wishing not to be bound by theory, the apparent viscosity or resistance to flow of drilling fluids is usually considered to be the result of two properties, plastic viscosity and yield point. Each of these two properties represents a different source of resistance to flow. Plastic viscosity is a property related to the concentration of solids in the fluid, whereas yield point is a property related to the interparticle forces. Gel strength, on the other hand, is a property that denotes the thixotropy of the fluid at rest. The yield point, gel strength, and in turn, the apparent viscosity of the fluid, commonly are controlled by PG,4 chemical treatments with materials such as complex phosphates, alkalies, mined lignites, plant tannins, and modified lignosulfonates.
It has been found that chromium modified lignosulfonates as well as mixed metal lignosulfonates of chromium and iron are highly effective as dispersants and therefore useful in controlling the viscosity of drilling fluids and in reducing the yield point and gel strength of the drilling fluids. Because chromium is potentially toxic, its release to the natural environment and the use thereof is continuously being reviewed by various government agencies around the world. Many of these agencies are either contemplating or have enacted stringent controls concerning the use of chromium-containing compounds in oil and gas well drilling fluids.